1. Field of the Invention
The present invention relates to processes for manufacturing an optical data storage disk stamper from an ion machinable ceramic substrate, and more particularly, to processes for replicating optical data storage disks using such stampers. The invention further relates to stampers manufactured by such processes and disks replicated using such stampers.
2. Description of the Related Art
Optical data storage disks are widely used, for example, as audio and video disks, e.g., compact disks, and in computer systems as part of compact disk read-only-memory devices for data storage and retrieval. An optical data storage disk may contain digital data in a spiral track of binary codes. These codes are formed on the disk as data patterns of minute pits and lands. FIG. 1 depicts an enlarged portion of a spiral track 1 from a compact disk (not shown) showing a sequence of pits 1a and lands 1b. In an audio disk, for example, the pits and lands in track 1 represent various types of binary codes, such as left-hand and right-hand stereo sound codes and codes which control disk reader motor speed and provide timings. Disk readers for decoding such tracks are well known.
For example, an optical data storage disk reader may rotate a disk with a diameter of about 125 mm at a playing speed of about 500 revolutions per minute at the center of the disk, at which a track begins, and at about 200 revolutions per minute at the end of the track near the outer edge of the disk. Nevertheless, the linear speed of the disk remains substantially constant, as it passes over an optical read-out device that decodes the track. Such optical read-out devices may include a configuration of mirrors and lenses, which directs a beam of light, such as a laser beam, at the spiral track. As the disk rotates, the directed beam may move outward from the center of the disk toward the disk's edge across the rotating track. When the beam is directed at a land portion of the track, it is reflected creating a light signal, and a photosensitive switch, such as a photosensitive diode, may be used to convert this reflected light signal into an electric signal. However, when the beam enters a pit in the track, it is not reflected, and no such electric signal is produced.
A conventional process for manufacturing an optical data storage disk stamper is described in U.S. Pat. No. 5,096,563 to Yoshizawa et al., which is incorporated herein by reference. FIGS. 2a-g are a schematic of cross-sectional illustrations showing steps in a conventional process for manufacturing an optical data storage disk stamper. In such a conventional process, a photoresist master disk includes a photoresist layer 3 deposited on the main surface of a glass substrate 2, such as plate soda-lime glass, as shown in FIG. 2a. Soda lime glass is made by fusion of sand with sodium carbonate or sodium sulfate and lime or limestone.
A laser beam La, which flickers according to a digital signal, exposes photoresist layer 3 to helically or concentrically form a data pattern 6 consisting of a latent image of a track of spots, e.g., pit locations. Either, a positive or a negative photoresist may be used. When a positive photoresist is used, areas exposed to light are removed by the development process. Conversely, when a negative photoresist is used, areas not exposed to light are removed by the development process. The exposed photoresist master disk then is developed to create a track of minute pits 3a corresponding to a digital signal to be recorded on the photoresist master disk, thus producing a developed master disk which has pit-carrying photoresist layer 3 and glass substrate 2, as shown in FIG. 2b.
Photoresist layer 3 of the developed master disk then is dried and fixed on glass substrate 2 to produce a dried master disk, as shown in FIG. 2c. A conductive metal, such as silver or nickel, may be sputtered on or applied by wet metalization to photoresist layer 3 to form a conductive film 4, rendering the surface of the developed master disk conductive and creating a mastering disk 4a having a multi-layered structure, as shown in FIG. 2d. Conductive film 4 may have a thickness of only a few molecules.
Mastering disk 4a then may be immersed in a nickel electroforming tank to plate conductive film 4 with nickel. As a result, a nickel layer 5, i.e., a nickel stamper, is formed, as shown in FIG. 2e. Nickel stamper 5 has a series of ridges 5a, each of which may be continuous or discrete and may correspond to one of pits 3a created in photoresist layer 3. Nickel layer or stamper 5 is separated from glass substrate 2, as shown in FIG. 2f, to create a negative die of the spiral track to be replicated on the optical data storage disk. Because nickel stamper 5 is extremely delicate, stamper 5 may be removed from glass substrate 2 by hand. Photoresist layer 3 (and conductive film 4) then may be removed from stamper 5, yielding nickel stamper 5 with a mold surface 6' bearing a negative image of data pattern 6, as shown in FIG. 2g. If conductive film 4 is formed from nickel, however, it may be left in place and may simply become part of stamper 5. After photoresist layer 3 (and conductive film 4) has (have) been removed from stamper 5, stamper 5 is rinsed, and a protective lacquer coating (not shown) may be applied to the surface of the negative die. The lacquer coating then may be cured, and the surface of stamper 5 opposite the negative die may be polished to remove any imperfections caused during the nickel plating.
After stamper 5 has been lacquered and polished, a hole may be punched in the center of stamper 5 in order to fix it to an injection molding apparatus. Punching the hole in stamper 5, however, may create stresses in the nickel and cause imperfections in the spiral track. Such stresses are unavoidable and nickel stamper 5 then is fixed in a mold of the injection molding apparatus. After the injection mold is closed, a thermoplastic resin, such as flowable polymethyl methacrylate, polycarbonate, acrylic resin, epoxy resin, unsaturated polyester resin, or the like, is injected onto the mold filling the track formed in stamper 5 with resin. After the resin has hardened, it is separated from stamper 5, providing an optical data storage disk replica having a face on which the binary code described by the data pattern is recorded.
A reflective material, such as aluminum or gold, including aluminum and gold alloys, may be applied to the data pattern face of replicas produced in this manner. Further, a protective lacquer film is coated on the reflective material, forming an optical disk. Two optical data storage disk replicas may be formed in this manner, bonded together, and subjected to a finishing process to produce a double-sided optical data storage disk.
The electroforming of a nickel stamper in conventional processes is a relatively time consuming procedure. Further, because of the delicate procedures associated with the electroforming step, current processes have not been fully automated. Manufacturing a conventional nickel stamper may take longer than about 180 minutes. The additional time required to manufacture stamper 5 by conventional processes makes such processes inefficient for replicating recently developed optical data storage disks which may involve small production quantities of a variety of types of audio/visual software. Moreover, current electroforming procedures employ toxic chemicals and require the disposal of hazardous materials, including solutions containing heavy metals, e.g., nickel. The lacquering procedure used as part of the finishing process also may produce significant quantities of toxic fumes and hazardous materials.
As mentioned above conventional stampers made from an electroformed layer of nickel are delicate and have limited lives. Repeated handling of conventional stampers may result in their deformation or other damage. Further, disk finishing procedures, e.g., lacquering, polishing, and hole punching, may result in the uneven application of lacquer and imperfection of or damage to the spiral tracks due to hole punching or polishing. Stampers suffering from such manufacturing imperfections or damage are discarded, and the manufacturing process is repeated to create new stampers. Finally, because electroformed nickel is susceptible to oxidation or pitting due, for example, to the presence of alkalis in the glass substrate when stored, nickel stampers are closely monitored to detect signs of such deterioration. Such stamper deterioration often can not be used. Therefore, stampers exhibiting oxidation or pitting or other physical damage are also discarded, and new stampers are manufactured to replace them. In addition, nickel stampers may produce stains on the disk replicas. Disks exhibiting such stains have little or no commercial value and are discarded, and therefore, such stains reduce the yield of the replicating processes.